1. Field of the Invention
The present invention relates to an electrophoretic display device and a method of fabricating the same. More particularly, the invention relates to an electrophoretic display device including a substrate with switching elements and driving electrodes, and an electrophoretic display element layer placed on the substrate, and a method of fabricating the device.
2. Description of the Related Art
In recent years, Liquid-Crystal Display (LCD) devices have been used extensively in a variety of fields. This is because LCD devices have the feature of reduced thickness and weight. Recently, the price of LCD devices has been lowered subsequent to the productivity improvements and at the same time, the screen size thereof has been able to be enlarged. Therefore, the application range of LCD devices has been expanding more and more. On the other hand, as the display device that realizes further thickness reduction than LCD devices and significant reduction of power consumption, electrophoretic display devices have been developed. Electrophoretic display devices, which utilize the electrophoresis phenomenon, i.e., the phenomenon that charged particles dispersed in a liquid are moved in the liquid in response to an electric field applied thereto from the outside, have several types. For example, with the microcapsule type electrophoretic display device, a transparent liquid, black minute particles electrically charged negatively (black pigment particles), and white minute particles electrically charged positively (white pigment particles) are enclosed in each of microcapsules. A pair of electrodes (i.e., a driving electrode and a common electrode) is arranged at each side of the microcapsule. By applying a voltage across the pair of electrodes, the black minute particles and the white minute particles are moved (or displaced) in the microcapsule, thereby displaying characters and images.
When the potential of the electrode (i.e., the common electrode) placed on the viewing side is positive, the black minute particles are attracted toward the said electrode and the white minute particles are moved toward the opposite side away from the said electrode. Therefore, the region of the said microcapsule is seen black. On the other hand, when the potential of the electrode placed on the viewing side is negative, the white minute particles are attracted toward the said electrode and the black minute particles are moved toward away from the said electrode. Therefore, the region of the said microcapsule is seen white. In this way, black and white (monochrome) characters and images can be displayed.
Moreover, the states of the black minute particles and the white minute particles are stable at their two positions adjacent to the pair of electrodes (i.e., the driving electrode and the common electrode) placed at each side of the said microcapsule, in other words, these particles are bistable (bistability). Thus, even if the application of the voltage is stopped, the states these particles are maintained unchanged (memory effect). Accordingly, it is unnecessary to continue the application of the voltage to keep the displayed characters and/or images, which makes it possible to reduce the power consumption significantly.
The size of the microcapsules is in the range of several tens micrometers (μm) to several hundreds micrometers (μm). Therefore, if the microcapsules are dispersed in a transparent binder, they will have an ink-like state and as a result, they may be coated on a plane by a printing method. For this reason, the binder containing the microcapsules dispersed therein may be termed the “electronic ink”.
The electrophoretic display device has several types other than the microcapsule type, such as the microcup type, electronic liquid powder type, and so on. The present invention relates to an electrophoretic display device of any of these types, which has an “electrophoretic display element layer”, in other words, a layer of electrophoretic display elements or a set of layer-shaped electrophoretic display elements. It is preferred that these electrophoretic display elements have bistability.
FIG. 1A is a plan view showing the schematic structure of a prior-art electrophoretic display device, and FIG. 1B is a cross-sectional view along the line IB-IB in FIG. 1A.
A prior-art electrophoretic display device 150 shown in FIGS. 1A and 1B comprises a TFT substrate 101 having Thin-Film Transistors (TFTs) and predetermined driving electrodes (both of which are not shown) arranged in a matrix array on the predetermined display region, a bistable electrophoretic display element layer 102 fixed on the display region of the TFT substrate 101, a transparent protection film 105 covered on the electrophoretic display element layer 102, and a sealing member 104 formed to surround the peripheries of the display element layer 102 and the protection film 105.
The bistable electrophoretic display element layer 102, which comprises a common electrode (not shown) formed on a main surface (back surface) thereof on the same side as the TFT substrate 101, is fixed on the display region of the TFT substrate 101 with an adhesive 103. The protection-film 105 is adhered to another main surface (front surface) of the display element layer 102 on the opposite side to the TFT substrate 101 with an adhesive 106. The protection film 106 covers entirely the said main surface of the display element layer 102. A person will see the characters and/or images displayed by the device 150 from the side of the protection film 106 (from the upper side in FIGS. 1A and 1B).
The periphery (side faces) of the layered structure formed by the electrophoretic display element layer 102 and the protection film 105 is covered with the sealing member 104. The purpose of the sealing member 104 is to prevent the moisture existing in the air and the air itself from entering the inside of the layer 102. Specifically, the entry of the moisture and the air into the layer 102 from the back surface (i.e., the lower surface in FIG. 1B) of the layered structure on the side of the TFT substrate 101 is prevented by the TFT substrate 101. The entry of the moisture and the air into the layer 102 from the front surface (i.e., the upper surface in FIG. 1B) of the layered structure on the opposite side to the TFT substrate 101 is prevented by the protection film 105. However, the respective side faces of the layered structure are kept opened. Therefore, the entry of the moisture and the air into the layer 102 from the side faces of the layered structure needs to be prevented by the sealing member 104. Since the plan shape of the layered structure is rectangular, the plan shape of the sealing member 104 is like a rectangular ring in FIGS. 1A and 1B.
The prior-art electrophoretic display device 150 having the above-described structure is fabricated in the following way.
Specifically, first, as shown in FIGS. 2A and 2B, the electrophoretic display element layer 102 having the adhesive 103 on its back surface is heated to a predetermined temperature, thereby softening the adhesive 103. Then, the display element layer 102 is placed and pressed on the display region of the TFT substrate 101 while aligning the position of the display element layer 102 with the said display region. In this way, the display element layer 102 is adhered onto the display region of the TFT substrate 101. Thereafter, a predetermined bubble elimination process (which is known) is carried out, thereby eliminating the bubbles remaining between the display element layer 102 and the TFT substrate 101.
Next, as shown in FIGS. 3A and 3B, the protection film 105 having the adhesive 106 on its back surface is adhered to the surface of the display element layer 102 while aligning the position of the protection film 105 with the said surface of the layer 2, thereby adhering the protection film 105 onto the display element layer 102. Then, a predetermined bubble elimination process is carried out again to eliminate the bubbles remaining between the protection film 105 and the display element layer 102.
Next, as shown in FIGS. 4A and 4B, a sealing material 110 is coated to have a belt-like shape around the entire periphery of the layered structure formed by the display element layer 102 and the protection film 105, thereby covering and sealing the whole side faces of the layered structure with the material 110. As the sealing material 110, an ultraviolet (UV) ray curing resin is preferably used. Spacers (not shown) in the form of minute particles are mixed and dispersed in the material 110. This is to facilitate the formation of the sealing material 110 having a desired height.
Subsequently, the entire sealing material 110 is irradiated with UV rays for curing. As a result, the sealing material 110 is turned to the sealing member 104 that has a rectangular ring-like plan shape and that seals the whole side faces of the layered structure formed by the display element layer 102 and the protection film 112. In this way, the prior-art electrophoretic display device 150 shown in FIGS. 1A and 1B is fabricated.
FIG. 5 is a schematic cross-sectional view showing an example of the internal structure of the bistable electrophoretic display element layer 102, where bistable electrophoretic display elements are of the microcapsule type.
As shown in FIG. 5, the electrophoretic display element layer 102 comprises an electronic ink layer 102a in which bistable electrophoretic display elements 102aa are uniformly dispersed, a common electrode 102b, and a transparent resin film 102c. The common electrode 102b is formed to entirely cover a main surface of the film 102c. The electronic ink layer 102a is formed to cover the whole surface of the common electrode 102b and therefore, the display elements 102aa are arranged over the whole surface of the electronic ink layer 102. The adhesive 103 is coated on the opposite main surface (back surface) of the electronic ink layer 102 to the film 102c. 
In addition, there are Patent Document 1 (Japanese Non-Examined Patent Publication No. 2005-309075) and Patent Document 2 (Japanese Non-Examined Patent Publication No. 2005-114820) as the prior-art references relating to the present invention.
The Patent Document 1 discloses an electronic ink display device and a method of fabricating the same. This device comprises a first substrate (TFT substrate) having display pixels; a second substrate formed on the first substrate, which has at least an electronic ink layer; and a protection substrate formed on the second substrate. The protection substrate is larger than the second substrate and is expanded from the second substrate. A sealing material is filled in the space between the first substrate and the protection substrate expanding from the second substrate (see claim 1 and paragraph 0006 of the Patent Document 1).
With this electronic ink display device of the Patent Document 1, the weakness of the mechanical strength of the overall electronic ink display device can be eliminated by the protection substrate and at the same time, the weakness of the electronic ink layer in the moisture resistance can be eliminated by the sealing material. In other words, a panel structure having improved mechanical strength and improved moisture resistance is obtainable (see paragraph 0007 and 0011 and FIGS. 1 and 2 of the Patent Document 1).
The Patent Document 2 discloses a microcapsule type electrophoretic display panel and a method of fabricating the same. This panel comprises a substrate (TFT substrate); a microcapsule display layer disposed on the substrate; a transparent resin film formed to cover the microcapsule display layer; and a transparent resin protection film formed to cover the transparent resin film. The transparent resin protection film protrudes laterally from a side face of the transparent resin film, and has a size larger than the transparent resin film in such a way as to form a gap between the transparent resin protection film and the substrate in the peripheries of the microcapsule display layer and the transparent resin film. A water-vapor shielding resin layer is filled in the gap (see claim 1 and paragraphs 0008 to 0009 of the Patent Document 2).
With the microcapsule type electrophoretic display panel of the Patent Document 2, the above-described gap is filled with the water-vapor shielding resin layer and therefore, the entry of moisture to the microcapsule display layer can be effectively prevented. This means that the characteristics of the microcapsule display layer can be effectively prevented from degrading. Moreover, since the protection function for the microcapsule display layer is significantly enhanced by the transparent resin protection film, the breakage of the microcapsule display layer due to external shock can be effectively avoided (see paragraphs 0010, 0011 and 0017 and FIG. 1 of the Patent Document 2).
By the way, with the above-described prior-art electrophoretic display device 150 shown in FIGS. 1A and 1B, the following display defects may occur:
The first display defect is blotches or smears appearing on the display screen. According to the inventor's research, it was found that the cause of this defect is moisture and/or air bubbles entered to the inside of the bistable electrophoretic display element layer 102. Specifically, a very small quantity of moisture and/or air bubbles exist in the gap between the display element layer 102 and the protection film 105, or in the gap between the sealing member 104 and the display element layer 102 and the protection film 105. The said moisture and/or air bubbles will enter the inside of the display element layer 102 during the fabrication process sequence, causing the first display defect.
The second display defect is that images are not displayed on the screen as desired even if the TFTs are driven, i.e., defective driving. Specifically, even if a predetermined voltage is applied across the driving electrodes on the TFT substrate 101 and the common electrode 102b of the display element layer 102 by driving the TFTs, the minute particles existing in the display elements 102aa will not be moved as intended and as a result, desired images will not be displayed on the screen. According to the inventor's research, it was found that the cause of this defect is moisture and/or air bubbles entered to the boundary between the TFT substrate 101 and the bistable electrophoretic display element layer 102. Specifically, a very small quantity of moisture and/or air bubbles remain in the gaps between the display element layer 102 and the protection film 105, between the sealing member 104 and the protection film 105, and between the sealing member 104 and the display element layer 102. The said moisture and/or air bubbles will enter the boundary between the TFT substrate 101 and the display element layer 102 during the fabrication process sequence, forming a narrow space between the TFT substrate 101 and the display element layer 102. Due to this narrow space, the second display defect will occur.